Multiple GPU card problems
Problems like this might happen when using different devices to render one frame. That's why render farms have always one type of GPU and/or CPU letting render one frame. If its an animation, lets say frame 1-100, just open Blender 2 times, let one instance of Blender render with one GPU the first half (frame 1-49) and the other instance with the other GPU the other half (frame 50-100).

I never used 2 GPUs though, i dont know if this will work perfectly. But i think so.